In this work, the melting line of platinum has been characterized both experimentally, using synchrotron X-ray diffraction in laser-heated diamond-anvil cells, and theoretically, using ab initio simulations. In the investigated pressure and temperature range (pressure between 10 GPa and 110 GPa and temperature between 300 K and 4800 K), only the face-centered cubic phase of platinum has been observed. The melting points obtained with the two techniques are in good agreement. Furthermore, the obtained results agree and considerably extend the melting line previously obtained in large-volume devices and in one laser-heated diamond-anvil cells experiment, in which the speckle method was used as melting detection technique. The divergence between previous laser-heating experiments is resolved in favor of those experiments reporting the higher melting slope.

A novel Y 3(1Àx) Er 3x Ga 5 O 12 nanocrystalline garnet has been synthesized by a sol-gel technique and a complete structural, morphological, vibrational, and optical characterization has been carried out in order to correlate the local structure of the Er 3+ ions with their optical properties. The synthesized nanocrystals are found in a single-phase garnet structure with an average grain size of around 60 nm. The good crystalline quality of the garnet structure is confirmed by FTIR and Raman measurements, since the phonon modes of the nano-garnet are similar to those found in the single crystal garnet. Under blue laser excitation, intense green and red visible and 1.5 mm infrared luminescences are observed, whose relative intensities are very sensitive to the Er 3+ concentration. The dynamics of these emissions under pulsed laser excitations are analyzed in the framework of different energy transfer interactions. Intense visible upconverted luminescence can be clearly observed by the naked eye for all synthesized Er 3+ -doped Y 3 Ga 5 O 12 nano-garnets under a cw 790 nm laser excitation. The power dependency and the dynamics of the upconverted luminescence confirm the existence of different two-photon upconversion processes for the green and red emissions that strongly depend on the Er 3+ concentration, showing the potential of these nano-garnets as excellent candidates for developing new optical devices. A IntroductionNowadays, rare earth (RE 3+ )-doped nanocrystals attract great attention due to their size, shape, and phase-dependent structural and luminescence properties, which make them suitable for fundamental and technological applications.1,2 On the other hand, the favorable physical and chemical properties of the oxide garnet crystals, such as high transparency from the UV to the mid-IR, high thermal conductivity, hardness, good chemical stability, and relatively low-energy phonons, make them one of the most important families of host matrices for the RE 3+ ions with interesting luminescence properties already used in lasers and phosphors. ions without charge compensation. 8,9From the point of view of the potential optical applications of the RE 3+ ions, one of the most interesting phenomena is their capacity to convert the infrared absorbed radiation into visible emitting light, known as energy upconversion.10 The particular selection of one or various RE 3+ ions and their concentrations allows controlling the upconverted visible luminescence to match a specific coordinate of colour, or even the generation of white light as a combination of red, green and blue (RGB) emissions. Thus there is an increasing demand for upconversion materials with important applications in upconversion lasers, due to the availability of powerful near-infrared commercial laser diodes, IR detection by conversion to visible light, where detectors are more efficient, and biological fluorescence labels and imaging or 3-D displays. 1,2,[11][12][13][14][15][16] When the RE 3+ ions are incorporated into the nanocrystals their upcon...

Room temperature angle dispersive powder x-ray diffraction experiments on zircon-type NdVO 4 were performed for the first time under quasi-hydrostatic conditions up to 24.5 GPa. The sample undergoes two phase transitions at 6.4 and 19.9 GPa. Our results show that the first transition is a zircon-to-scheelite-type phase transition, which has not been reported before, and contradicts previous non-hydrostatic experiments. In the second transition, NdVO 4 transforms into a fergusonite-type structure, which is a monoclinic distortion of scheelite-type. The compressibility and axial anisotropy of the different polymorphs of NdVO 4 are reported. A direct comparison of our results with former experimental and theoretical studies on other rare-earth orthovanadates found in literature highlights the importance of the role played by non-hydrostatic stresses in their high-pressure structural behavior.

The 5d transition metals have attracted specific interest for high-pressure studies due to their extraordinary stability and intriguing electronic properties. In particular, iridium metal has been proposed to exhibit a recently discovered pressure-induced electronic transition, the so-called core-level crossing transition at the lowest pressure among all the 5d transition metals. Here, we report an experimental structural characterization of iridium by x-ray probes sensitive to both long- and short-range order in matter. Synchrotron-based powder x-ray diffraction results highlight a large stability range (up to 1.4 Mbar) of the low-pressure phase. The compressibility behaviour was characterized by an accurate determination of the pressure-volume equation of state, with a bulk modulus of 339(3) GPa and its derivative of 5.3(1). X-ray absorption spectroscopy, which probes the local structure and the empty density of electronic states above the Fermi level, was also utilized. The remarkable agreement observed between experimental and calculated spectra validates the reliability of theoretical predictions of the pressure dependence of the electronic structure of iridium in the studied interval of compressions.

In this paper, we present an ab initio study within the framework of density functional theory employing the generalized gradient approximation applied to the study of the structural, elastic, and electronic properties of yttrium gallium garnet, Y 3 Ga 5 O 12 , under hydrostatic pressure. The calculated structural ground state properties are in good agreement with the available experimental data. Pressure dependence of the elastic constants and the mechanical stability are analysed up to 90 GPa, showing that the garnet is mechanically unstable above 84 GPa. We also present the electronic band structure calculations which show that upon compression the fundamental direct gap first increases up to 63 GPa and later monotonically decreases under pressure. V C 2013 AIP Publishing LLC.

Iron oxides are among the major constituents of the deep Earth’s interior. Among them, the epsilon phase of Fe2O3 is one of the less studied polymorphs and there is a lack of information about its structural, electronic and magnetic transformations at extreme conditions. Here we report the precise determination of its equation of state and a deep analysis of the evolution of the polyhedral units under compression, thanks to the agreement between our experiments and ab-initio simulations. Our results indicate that this material, with remarkable magnetic properties, is stable at pressures up to 27 GPa. Above 27 GPa, a volume collapse has been observed and ascribed to a change of the local environment of the tetrahedrally coordinated iron towards an octahedral coordination, finding evidence for a different iron oxide polymorph.

This work reports an experimental and theoretical lattice dynamics study of nanocrystalline Y 3 Ga 5 O 12 (YGG) garnet at high pressures. Raman scattering measurements in nanocrystalline Tm 3+ -doped YGG garnet performed up to 29 GPa have been compared to lattice dynamics ab initio calculations for bulk garnet carried out up to 89 GPa. Good agreement between the theoretical vibrational modes of bulk crystal and the experimental modes measured in the nanocrystals is found. The contribution of GaO 4 tetrahedra and GaO 6 octahedra to the different phonon modes of YGG is discussed on the basis of the calculated total and partial phonon density of states. Symmetries, frequencies, and pressure coefficients of the Raman-active modes are discussed. Moreover, the calculated infrared-active modes and their pressure dependence are reported. No pressure-induced phase transition has been observed in nano-YGG up to 29 GPa. This is in agreement with theoretical results, which show a mechanical instability of YGG above 84 GPa, similar to what occurs in Gd 3 Ga 5 O 12 .

The effects of pressure on the crystal structure of aurophilic tetragonal gold iodide have been studied by means of powder X-ray diffraction up to 13.5 GPa. We found evidence of the onset of a phase transition at 1.5 GPa that is more significant from 3.8 GPa. The low-and high-pressure phases coexist up to 10.7 GPa. Beyond 10.7 GPa, an irreversible process of amorphization takes place. We determined the axial and bulk compressibility of the ambient-pressure tetragonal phase of gold iodide up to 3.3 GPa. This is extremely compressible with a bulk modulus of 18.1(8) GPa, being as soft as a rare gas, molecular solids, or organometallic compounds. Moreover, its response to pressure is anisotropic.